1. Field of the Invention
This invention relates to ion mobility spectrometers and more particularly to controlling selected ions therein by selective sequencing of linear and non-linear electric fields.
2. Description of the Prior Art
The techniques of using ion mobility at atmospheric pressure for spectrometry has been well established. The major parts of an ion mobility spectrometer (IMS) device are: the sample inlet, ionization region, shutter grid, drift region and ion detector. In conventional IMS, a uniform or linear field, for example, 200 V per centimeter, is applied across the drift region and the various ions upon release by the shutter grid are allowed to drift through the drift region toward the ion detector. After release by the shutter grid, the ions of a particular mobility, K.sub.0, are located at a particular location in the drift region at each instant of time. The ion detector provides a signal indicative of the number of ions arriving at a collector plate and the time lapse from the time the shutter grid was pulsed open to the time the ions arrive at the collector is an indication of the mobility of the ions collected.
The electric field in the drift region of an ion mobility spectrometer may be provided in two ways. The first way is described in U.S. Pat. No. 4,378,499 which issued on Mar. 29, 1983 to G. E. Spangler, D. N. Campbell and S. Seeb wherein the drift region having a longitudinal axis is surrounded by a plurality of concentric conductive rings 22 separated from one another by a plurality of insulating rings 23 which are secured into a unitary cylindrical body. A voltage divider 24 connected across the output of a high voltage bias source 25 applies progressively increasing voltages to the conductive rings 22 thereby creating the electric field. A second way is shown in U.S. Pat. No. 4,390,784 which issued on June 28, 1983 to D. R. Browning et al. In U.S. Pat. No. 4,390,784 a tube 14a has a thick film resistor 34 coated on its inside surface. A voltage potential difference represented by 36a and derived from high voltage source 36 is impressed across resistor 34 thus causing an electrostatic drift field in drift region 14 which moves ions toward an ion detector. Tube 14a is cylindrical in shape.